Bacteria and viruses are the ‘germs’ that make you sick. But not all bacteria are bad. Some are actually beneficial, keeping us healthy and helping us to digest our food. Such bacteria are found in yogurt, and are sometimes called ‘good’ bacteria. Good and bad are not the only ways that bacteria are classified. More scientifically, they are classified by their arrangement and shape.

There are three main bacterial shapes: coccus (round), bacillus (rod), and spirillus (spiral). There are also three common arrangements. Bacteria found in chains are said to be strepto-, while those found in clusters are called staphylo-. Bacteria said to be diplo- are found in pairs.


  • Plain yogurt (with active cultures)
  • Water Dropper
  • Toothpick
  • Microscope Slide and coverslip
  • Compound microscope


Use the toothpick to thinly spread a small amount of yogurt on the slide. Place a drop of water over the yogurt and add a coverslip. View the slide on low power to find an area where the yogurt is very thin. Switch to high power and see how many kinds of bacteria you can find. One common type found in yogurt is lactobacillus acidophilus, so named because the bacteria are rod-shaped and produce lactic acid.

microscopic bacteria succession in milk